Early studies on the bacterial content of milk were made mainly to satisfy the interst of people who wished to determine the various materials that contained bacteris. Soon the value of bacterial counts, as an indication of the general conditions of production, of handling, and of the keeping qualities of milk, became evident and bacterial counts were used to obtain information concerning these problems. Numbers of bacteria in milk have been used also in the studies of the desirable and undesirable changes in milk. In the last few years our citizenry has been made more importance of bacteria in milk. As the number of milk dealers have increased and our population in the cities has become more conjested, more stringent regulation of our milk supplies has been practiced. Of major importance in this regulatory program is the bacterial count of milk. Large dairy manufacturing plants, which have also recognized the importance of high bacterial counts in influencing the quality of their products, have encouraged production of low count milk even to the extent of giving bonuses to such producers and rejecting milk that did not come within their standards.

The house fly, Musca domestica can transmit human pathogens including Escherichia coli O157:H7 through regurgitation of ingested bacteria from the crop which is a foregut organ of house fly and stores the excess ingested nutrients. Interactions between the ingested bacteria and the crop have a direct influence on bacteria persistence, survival and ultimately fly vector competence. In this research, in situ crop vessel assay was developed to investigate bacterial growth within fly crops up to 48 hours post-ingestion. Flies were fasted for 12 h prior to feeding E. coli O157:H7 pEGFP and then fed bacteria with red food color which was added to confirm that flies had consumed the bacteria. After feeding, flies with red abdomens were aseptically dissected and crops were removed and maintained in sterile phosphate buffered saline in microtiter plates held at 32˚C. For each time point (0, 24 and 48 hours post-ingestion), five crops were homogenized individually using a tissue grinder and bacterial levels (CFU/crop) were monitored using plate counts. Confocal microscopy of intact crops was used to monitor biofilm development. There was no statistical difference in cell numbers (CFU/crop) over the 48 h incubation period. Microscopy showed that upon prolonged incubation, GFP-expressing E. coli within the crop produced biofilms. This method showed greater reproducibility in studying crop bacteria level than using a live fly feeding study. But this system was not recommended to study the interaction between bacteria and the crop of housefly.